Dr. Sean Wheatley, PhD – Science and Research Lead/29 November 2019
The impact of being overweight or obese on health is well known, but some of the most widely used ways of measuring this are often criticised. Finding a simple to use and easy to understand marker of obesity can be an important step in helping an individual understand their own health risk, and one possible solution is the waist to height ratio (WHtR).
What’s wrong with using body weight or BMI?
Both body weight and body mass index (BMI) are limited because they don’t take into account how much of us is fat, muscle, bone etc. They also don’t give an idea of where any fat on our body is stored. This is an important limitation, as WHERE fat is stored can be more important than how much fat there is.
Can’t we just use waist circumference?
Waist circumference (WC) does help to address this problem. The only bones in this region are part of the spine, and the muscles in this area are not those which are likely to get significantly bigger or smaller based on physical activity and weight lifting etc. Any difference in someone’s waist size is therefore likely to reflect fat storage. Fat stored around the middle is more detrimental to health than fat stored around the thighs and bum (see below), so getting a good indication of this can be very useful.
The problem is, taller people tend to have larger WCs. You wouldn’t expect a lean, healthy individual who is 6 foot 5 inches (about 196 cm) to have the same WC as a lean, healthy individual who is 5 foot 2 inches (about 157 cm); waist size will increase proportionately in the same way arm length or head size would. That means using waist circumference on its own doesn’t necessarily just tell us how much fat is being stored after all, we need to take into account the effect of height. That is where the WHtR comes in.
So what is waist to height ratio?
WHtR is, very simply, your waist size* (in centimetres) divided by your height (in centimetres). This takes the benefits of using WC and addresses one of the major limitations by helping to correct for height. The result is a better indicator of how much fat is stored around the middle!
How come fat stored around the middle is worse anyway?
Fat can either be stored under the skin (subcutaneous fat) or it can be stored in and around internal organs (visceral fat). The fat stored under the skin, for example on the arms, legs or bum, can be unsightly if there is an excess of it; but it doesn’t affect the function of the body in the same way visceral fat can do.
Fat stored around the middle can include both subcutaneous fat and visceral fat. The visceral fat is particularly harmful, as it can impact on the function of the essential organs such as the liver and the pancreas. It also creates and excretes a number of potentially dangerous substances (called adipokines) which can cause inflammation and other issues. Even the subcutaneous fat in this area has been shown to be more detrimental to health than that stored elsewhere, as it produces more of these adipokines than fat in other parts of the body does. Altogether, fat stored around our middle is very bad for us; when it comes to body shape it’s better to be a pear than an apple!
So back to waist to height ratio, what makes it so useful?
As well as being simple to work out there are another couple of things that make WHtR a useful option:
* The same cut points seem to apply to males and females, young and old, and across different ethnicities. This isn’t true for weight, BMI or WC; with different classifications being needed for lots of different groups
* This universal cut point is 0.5, which translates really well to a simple take home message; try and keep your waist size to less than half your height
What if I don’t have a tape measure?
Whether you have a tape measure or not there is a really simple way to tell if your waist size is less than half your height (i.e. less than 0.5), the string test. Simply:
1. Cut a piece of string so that it is as long as you are tall (you may need some help with this, and it is easier to do it if you stand against a wall)
2. Fold the piece of string in half
3. Wrap the folded piece of string around your waist*. If the ends touch or overlap, your WHtR is less than 0.5!
What if my waist is more than half my height?
Importantly, it is possible to address this through lifestyle changes. Making some changes to your dietary approach might be a good place to start for many, whilst exercise and other physical activity can help to reduce waist size even if body weight doesn’t change. Improving sleep and reducing stress can also help.
So what’s the bottom line?
Waist to height ratio can be a useful tool for monitoring your health. It is simple to use and understand, and it addresses some of the key issues that using body weight, BMI or WC have. Using the string test can make it even simpler, with the overall message being: try and keep your waist size to less than half your height!
* Identifying where your waist is properly can be the hardest part of using WC or WHtR. It ISN’T where you wear your trousers! Your true waist is half way between the top of your hips and the bottom of your ribs (you should be able to find bony bits sticking out at both of these points). Other methods include measuring one finger width above your belly button (if the position of your belly button hasn’t dropped) or finding the narrowest point between your hips and ribs (bending slightly to the side can help you find an “indent”). You should take any measurements whilst you are relaxed and at the end of breathing out (not whilst “sucking it in”!). Whichever point you identify as your waist it is important you are consistent, so if you take any other measurements in the future it is a fair comparison.
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